The present invention relates to pressure-sensitive adhesives and to articles containing such adhesives.
Pressure-sensitive adhesives are commonly used in the preparation of numerous articles, including adhesive tapes and other systems wherein ready attachment of two components under only an applied pressure is desirable. The performance of pressure-sensitive adhesives comprises a balance of three properties: peel resistance (a measure of adhesion), tack and shear strength (a measure of cohesion). In general, any system perturbation which serves to improve peel and tack results in a depletion of shear strength. Conversely, any change which improves shear strength has the effect of hurting peel and tack. In addition, other properties, such as clarity and color stability of the ultimate film prove to be of importance.
A wide variety of polymeric compositions have been found to be useful as pressure-sensitive adhesives when applied to a substrate. Polymers may be applied in several forms, as solutions, emulsions or as hot melts. The key property for a number of these applications is the attainment of very high shear strength. A number of methodologies have been described which allow for the preparation of polymeric systems which achieve this high shear strength. In general, these methodologies suffer from reduction in peel and tack which accompanies this enhanced shear. Increases in polymer molecular weight may serve to improve shear strength at the expense of peel and tack. Incorporation of highly polar monomers, such as polymerizable carboxylic acids, or the inclusion of cross-linking monomers, such as N-methylol acrylimide, may also provide increased shear strength with the concomitant loss of peel and tack.
An approach which has been extensively documented is that of incorporation of metal crosslinks into the adhesive composition. U.S. Pat. No. 4,540,739 describes the use of alkali metal hydroxides as neutralizing agents in the preparation of high shear pressure-sensitive adhesives. Treatment with polyvalent metals to prepare crosslinked films has been described in U.S. Pat. No. 2,754,280. However, peel and tack are compromised.
U.S. Pat. No. 4,812,541 describes a high performance pressure-sensitive adhesive composition which includes a combination of glycidyl and N-vinyl lactam monomers. The resulting polymer system was found to exhibit a good balance of adhesion to aluminum and high cohesion.
A recent disclosure, U.S. Pat. No. 4,759,983, describes the use of a functional monomer of the following formula: ##STR1## in which R.sub.1 is a divalent organic radical at least 3 atoms in length, and X is organoacyl or cyano. Incorporation of such monomers was found to lend improved shear strength to adhesive polymer compositions without significant compromise of peel and tack. All examples of U.S. Pat. No. 4,759,983 are run at a pH from 4 to 6.5 adjusted with either ammonia or sodium hydroxide. U.S. Pat. No. 4,908,403 discloses treating the above systems with an acid-containing component to further increase the shear strength of the resultant adhesive polymer. U.S. Pat. No. 4,908,403 states a preference for acidic pH values.
In U.S. 4,540,739 data is presented which shows that neutralizing adhesive emulsion polymers to a pH of 8.5 to 10, with non-permanent bases such as ammonium hydroxide, does not affect the shear strength of the resultant adhesive film. Neutralization with sodium hydroxide produced significant improvement in shear.